Method of treating chemical pulf



March 13, 1934.

R. s. HATCH 1,951,017

METHOD OF TREATING CHEMICAL PULP Filed July 20, 1933 VENTOR' ATTORN EYSPatented Mar. 13, 1934 UNITED STATES PATENT OFFICE METHOD OF TREATINGCHEMICAL PULP Application July 20, 1933, Serial No. 681,251

9 Claims.

This invention relates to the preparation of chemical paper pulp andparticularly to an improved method of treating pulp to facilitatebleaching thereof and a product of enhanced whiteness and betterphysical properties.

Chemical paper pulp is produced by cooking Wood chips in variouschemical solutions. The present invention can be applied to pulpsproduced by any of the well known processes. It is particularlyapplicable to pulps produced from coniferous woods, but it may beutilized in treating pulp from any source.

The fibres or cells which make up the original wood structure may bedivided roughly'into two classes, longitudinal fibres and medullary rayfibres. The longitudinal fibres are those which are arrangedsubstantially parallel to the vertical axis of the tree and which makeup the main cellulosic component of the wood. The medullary ray fibresare short fibres which in the original wood are substantiallyperpendicular to the axis of the tree. They serve as binders for thelongitudinal fibres and .as conductors of sap from the inner bark to thelongitudinal fibres.

When the wood substance is resolved by chemical agents, the bulk of thenon-cellulosic wood substance, generally referred to as lignin isdissolved out and the remaining chemical wood pulp is made up of a moreor less pure fibre mixture consisting of longitudinal fibres andmedullary ray fibres. The proportion of these two constituents isroughly 90-92% by weight of longitudinal or long fibres, and 840% byweight of medullary ray fibres. the longitudinal fibres vary in length,depending upon the wood species, from about 2.8 millimeters to about 4.5millimeters. The medullary ray fibres are readily distinguishablebecause they are much shorter than the longitudinal fibres, being about1/25 to 1/30 of the length thereof, and having about the diameter of thelongitudinal fibres. v

The medullary ray fibres are much morere-r sistant to the action of thecooking liquor than,

are the longitudinal fibres. It is much more difficult to remove thenon-cellulosic material from the medullary ray fibres. Whether thisisdue to the manner in which the lignin or non-cellulosic constituents arecombined withthe Il'lfid', ullary ray fibre substance is not known. Ihave discovered, howeven-that after cooking the med- "ullary ray fibrescontain a larger proportion of non-cellulosic constituentsandparticularly coloring elements than the longitudinal fibres. In

In coniferous woods,-

order to bleach the medullary ray fibres it is necessary to subject themto a much more dras tic bleaching treatment than is required to whitenthe longitudinal fibres. If, therefore, the pulp is bleached to asuitable whiteness, the lonat; gitudinal fibres are considerablyweakened and the physical properties of the pulp are deterio-= rated.

It is the object of the present invention to provide a method oftreating chemical paper as pulp so as to facilitate the bleachingoperation and to produce a brighter pulp having improved physicalproperties.

Another object of the invention is to provide a method of removing themedullary ray fibres from the longitudinal fibres before the pulp isbleached, whereby coloring elements are separated from the pulp andbleaching is accomplished'more readily and with a lower consumption ofbleaching agents.

Other objects and advantages of the invention will be apparent as it isbetter understood by reference to the following specification andaccompanying drawing, in which the figure represents diagrammatically anapparatus adapted for 30 use in separating the medullary ray fibres fromthe unbleached chemical paper pulp.

In carrying out the invention, pulp may be produced by any of the wellknown chemical processes. After the completion of the cooking, the pulpis washed to removethe chemical cooking agent and is screened to removeknots and shieves or unresolved wood particles in the usual manner. Thepulp as thus prepared consists of longitudinal fibres and the shortmedullary ray fibres, and is then subjected to the procedure hereinafterdescribed to separate the medullary ray fibres from the longitudinalfibres.

In carrying out the invention, the pulp is first diluted to aconsistency of approximately .2% to 3% by weight with water, and it isthen caused toflow over a suitable separating medium, for example, ascreen of wire cloth having about 25 meshes to an inch. In thus flowingover the screen, the large excess of ater carries the short 10omedullary ray fibres thrcugh'the screen and effectively separates them.From 90 to 95% of the short medullary ray fibres may be removed thusfrom the pulp, leaving the longitudinal fibres which may be deliveredto, a suitable conveyor the short medullary ray fibres from the pulp.

Water may be sprayed uponthe pulp as it passes over the separatingscreen, although dilution of the pulp as indicated is usually sufiicientto ac- 5 complish the desired purpose.

The separation of the medullary ray fibres from the pulp removes aconsiderable proportion of coloring elements from the original pulp.This is shown readily by the improved whiteness of the pulp from whichthe medullary ray fibres have been removed, and also by the color of themedullary ray fibres which have been separated. The medullary ray fibresmay be collected from the water in which they are suspended and formedinto sheets. Such sheets show a distinctly darker tone than the originalpulp, whereas sheets formed from the longitudinal fibres display adistinctly lighter tone than the original pulp.

The improvement can be demonstrated, moreover, by definite testsindicating bleachability and lignin content. As an example, a pulpsubjected to the test for bleachability with chlorine gave the followingresults: 26 Per cent (0) Original pulp -1 5.54 (b) Longitudinal fibres5.12 (c) Medullary ray fibres 7.8

o This pulp, when tested for lignin content, gave the following results:

' Per cent (41) Original pulp 2.54 (b) Medullary ray fibres 5.10

Uniractlonated Fractionated 0 Mullen. 108 113 Teen-.. 2. 2. Fold 1 72Alpha per cent 88. 36 89. 41 Copper No 2. 20 2. 08 0 It will be observedthat in the fractionated pulp every characteristic of the product hasbeen improved as the result of,the separation of the short medullary rayfibres.

The fractionated pulp may be subjected to any .of the .usual bleachingoperations. The details of such operations are well known and form nopart of the present invention. The bleached pulp .can. be brought to ahigh degree of "whiteness closely approximating a complete absence ofcolor, without reducing the strength and other desirable qualities ofthe product.

It is evident from the foregoing tests that the longitudinal fibresfrom.',which the medullary ray .5 fibres have been separated may bebleached with a smaller proportion of bleaching agent, and particularlyneed not be subjected to'the more drastic treatment necessary toremovecoloring elements from the medullary ray fibres. The deteriorationof the longitudinal fibres through excessive bleaching necessary tosecure'a white product is thus avoided by the expedient of removingmedullary ray fibres before the bleaching operation. The reduced 'costof bleaching and I6 the resulting improvement in the product compensatefor a loss of upward to 10% of the original pulp, even where the shortmedullary ray fibres are discarded.

The short medullary ray fibres may be bleached after separation by anyof the well known bleaching processes. It is possible to utilize theshort medullary ray fibres after separation for the production of paperstock, or for mixing with various pulps to secure the desired results,such as a quick-beating pulp, since the fine material hydrates much morereadily than the longer fibres. The short medullary ray fibres may beused also as a raw material for"the preparation of alpha cellulose andin the viscose and cellophane industries.

As anexample of a suitable apparatus for the practice of the invention,5 indicates a trough from which the pulp, previously diluted toapproximately .2% to .3%, is permitted to overflow onto. screens 6 whichare disposed at a suitable angle, for example about 45. The angle ofinclination'is not important except that it must be sufiicient to permitthe pulpto fiow freely but not too rapidly. The screens may consist ofwire mesh cloth having about 25 meshes per inch. In the practicalapparatus, the screens may be about 8 feet from the trough 5 to trough 7in which the pulp is collected and from which it may be withdrawn byscrew conveyors 8. Pipes 9 may be provided to spray water onto thescreens 6, particularly near the lower ends thereof; The water whichdilutes the pulp and any additional spray water passes through thescreens 6 carrying with it the short medullary ray fibres, so that thepulp which accumulates in the trough '7 consists substantially oflongitudinal fibres only. The water with the short medullary ray fibresis collected in a receptacle 10 beneath the screens and may be withdrawnthrough a pipe 11 and delivered to suitable settling or separating tanks12. If desired, pipes 13 may be provided to spray water 1 into thetrough 7 which may be perforated to Iii) permit the escape of the waterwith any residual pass therethrough, while the longer longitudinalfibers pass thereover.

2. The method of treating cellulosic pulp to separate the medullary rayor other fibers of similar length, from the longer longitudinal fibers,which" consistinfiowing the pulp, having a consistency of approximately0.2 to 0.3 percent, downwardly over an inclined screen, the inclinationof the screen and the mesh thereof being so correlated that themedullary ray, and other short fibers, will pass therethrough, whilethelonger longitudinal fibers will pass thereover..

3. The method of treating unbleached cellulosic pulp, which comprisesseparating the medullary ray, or other fibres of similar length from thelonger longitudinal fibres of the pulp by the method which consists inflowing the pulp downwardly-over an inclined screen, the inclination ofthe screen and the mesh thereof being so correlated that the medullaryray, and other fibers of similar length, will pass therethrough, whilethe longer longitudinal fibers will pass 15c thereover, and thereafterbleaching the pulp consisting of substantially only the longerlongitudinal fibers.

4. The method of treating unbleached cellulosic pulp, to separate themedullary ray, or other fibers of similar length, from the longerlongitudinal fibers of the unbleached pulp, which consists in flowingthe pulp downwardly over an inclined screen, the inclination of thescreen and the mesh thereof being so correlated that the medullary ray,and other short fibers, will pass therethrough, while the longerlongitudinal fibers will pass thereover.

5. The method of treating unbleached cellulosic pulp to separate themedullary ray, or other fibers of similar length, from the longerlongitudinal fibers of the unbleached pulp, which consists in flowingthe pulp, having ,a consistency of approximately 0.2 to 0.3 percent,downwardly over an inclined screen, the inclination of the screen andthe mesh thereof being so correlated that the medullary ray, and othershort fibers, will pass therethrough, while the longer longitudinalfibers will pass thereover.

6. The method of improving cellulosic pulp for bleaching, whichcomprises removing the medullary ray, or other fibers of similar length,and colloidal matter from the longer longitudinal fibers by the methodwhich consists in flowing pulp downwardly over an inclined screen, theinclination of the-screen and the mesh thereof being so correlated thatthe medullary ray, and other short fibers, and the colloidal matter willpass therethrough, while the longer longitudinal fibers will passthereover.

7. The method of improving cellulosic pulp for bleaching, whichcomprises removing the medullary ray, or other fibers of similar length,and

the mesh thereof being so correlated that the medullary ray, and othershort fibers, and the colloidal matter will pass therethrough, while thelonger longitudinal fibers will pass thereover.

8. The method of treating cellulosic pulp, which comprises separatingthe medullary ray, or other fibres of similar length from the longerlongitudinal fibres of the pulp by the method which consists in flowingunbleached pulp downwardly over an inclined screen, the inclination ofthe screen and the mesh thereof being so correlated that the medullaryray, and other fibers of similar length, and colloidal matter will passtherethrough, while the longer longitudinal fibers will pass thereover,and thereafter bleaching the pulp consisting of substantially only thelonger longitudinal fibers.

9. The method of treating cellulosic pulp, which comprises separatingthe medullary ray, or other fibres of similar length from the longerlongitudinal fibres of the ..pulp by the method which consists inflowing unbleached pulp, having a consistency of approximately 0.2 to0.3 percent, downwardly over an inclined screen, the inclination of thescreen and the mesh thereof being so correlated that the medullary ray,and other fibers of similar length, and colloidal matter will passtherethrough, while the longer 1ongitudinal fibers will pass thereover,and thereafter bleaching the pulp consisting of substantially only thelonger longitudinal fibers.

RAYMOND S. HATCH.

